In conventional compact disc players, the surface of the disc bears a number of substantially circular record tracks, each composed of a row of pits or bumps indicative of a recorded information signal. The signal recorded in a track is reproduced by irradiating the record track with a coherent light beam, advantageously a laser beam, which is then reflected from the disc surface in a modulated condition indicative of the recorded signal. The modulated reflected beam is directed to fall on a photosensor device, which generates electrical signals in response to the amount of light received. The recorded signal may then be recovered from these electrical signals.
An optical pickup is provided in the compact disc player for generating the laser beam, focusing it onto the disc surface, and directing the modulated reflected beam to the photosensor device. Conventionally, the laser beam is split into three parallel beams, a main beam and two auxiliary beams, which are focused through an object lens onto the disc surface. The main beam is focused directly onto the record track and is used for reading the recorded signal and for focal servo control. The auxiliary beams are focused along the record track ahead of and behind the main beam and are used for tracking servo control. The reflected beams pass back through the object lens and are then directed to the photodetector or photosensor device for processing and reproducing of the information signal as represented in the modulated beams. The photosensor device within the optical pickup thus is used not only for light detection in the modulated main beam to reproduce the information signal, but is also used for light detection in the main and auxiliary beams for focal servo control and tracking servo control in a conventional three beam tracking system. Consequently, the photosensor has a plurality of photosensitive elements, each receiving different portions of the reflected beams.
The compact disc player is contained in a housing or casing generally in a shape of a rectangle. If the optical pickup is moved parallel to one side of the rectangle, this side must be at least as long as the diameter of the disc within plus one half the width of the object lens in order to accommodate the optical pickup at its outermost position with the object lens centered over the outermost record track.
With the growing popularity of compact disc players, the trend has been towards reducing the size of the casing as much as possible. An advantageous compact disc player is disclosed in the commonly assigned U.S. patent application Ser. No. 06/603,370, referred to above, whose disclosure is herein incorporated by reference. The compact disc player disclosed therein includes an optical pickup whose structure and mounting allow the casing to be reduced to a minimum size. The optical pickup shown therein is moved radially with respect to the disc, as is conventional, but in a direction forming an acute angle with two opposed sides of the rectangular casing. The casing then takes the shape of a square with the disc inscribed therein.
When the optical pickup is thus driven in this acutely angled direction, the optical paths of the three beams must be altered. To understand this, it must be appreciated that in order to use the main beam and the two auxiliary beams as described, the three beams must be focused on the surface of the disc to lie along a straight line substantially tangential to the record track or at a small constant angle relative thereto. As will be described in detail below, this requires that the three beams be rotated by a predetermined angle about the optical axis of the main beam. Physically, the three beams are emitted parallel to each other and parallel to the surface of the disc, but lie in a plane which is tilted so as to intersect the surface at a predetermined angle. When the three beams are reflected from the surface of the disc and reformed parallel to the disc surface, they again lie in the tilted plane. Therefore, it is necessary that the photosensor device be mounted with the photosensitive elements in a plane at the same predetermined angle in order to properly detect the reflected beams.
Advantageously, the photosensor device is an integrated circuit and the photosensitive elements are semiconductor portions therein. The photosensor integrated circuit or chip is fixed in a relatively large, transparent package mounted in turn on a circuit board. Each photosensitive element has a pair of terminal leads extending therefrom with external portions extending outside of the package for connection to the focal and tracking servo control circuits also mounted on the circuit board. Since the photosensor device must be mounted at the above-mentioned predetermined angle relative to the surface of the disc, it is necessary to make a corresponding change in the wiring pattern of the circuit board. However, this significantly increases the space requirement for the package bearing the photosensor chip, thus limiting the reduction in size of the compact disc player.